Recent research and development activity has focused on paste-form, gel-like and thixotropic forms of liquid automatic dishwasher detergents because of the desirable advantages to the consumer over conventional powdered forms of detergents. These advantages include ease of dispensing from the container, lower volume consumption per wash load because of higher concentrations of active ingredients, and long term storage without moisture spoilage.
The development of suitable liquid automatic dishwasher detergents has not been without its problems due to the requirement that such formulations incorporate a number of ingredients which are generally incompatible, i.e., they tend to react with each other prior to use in the dishwasher. Additionally, liquid automatic dishwasher detergents should also exhibit thixotropic properties, i.e., it should be highly viscous in a quiescent state and have relatively high yield values (Bingham Plastic), but when subjected to a shear stress, such as being squeezed through a orifice, it should have flow properties similar to a viscous liquid so it can be easily dispensed into a dishwasher detergent dispenser cup. Once inside the dispenser cup it should quickly revert to the high viscosity/Bingham plastic state.
Another common problem with liquid automatic dishwashing detergent compositions is that they tend to separate into substantially solid and liquid phases during the shelf life of the product. Improvement in the phase stability has been accomplished by the addition of clays, polymers, soaps or combinations of clays and soaps. However, a drawback of this approach is that any addition to the detergent composition may adversely affect the rheological properties of the detergent composition.
Similarly, addition of a fragrance to a liquid automatic dishwasher detergent to impart a desirable scent has also been problematical because of the effect on the complex balance of ingredients within the composition necessary to produce the desired rheology and other physical characteristics. For example, the presence of a chlorine releasing compound would be expected to have a detrimental effect on an oil-type fragrance that is readily oxidized. In addition, the presence of oil-like fragrances would be expected to have an adverse effect on the phase stability of such compositions because of their tendency to act as defoamers.